The axons of dorsal column nuclear projection neurons exit the nuclei and are referred to as the internal arcuate fibers as they sweep ventrally and then medially to cross the midline at the same medullary level as the nuclei. Immediately after crossing the midline, these fibers form the medial lemniscus (see
Fig. 4.6C–E ), which projects rostrally to the thalamus. Knowledge of the level of this decussation is clinically important because damage to the dorsal column–medial lemniscal pathway below this level, which includes all of the spinal cord, will produce loss of fine somatosensory discriminatory abilities on the same, or ipsilateral, side of the lesion, whereas lesions above this level will produce contralateral deficits. Moreover, because there is a clear somatotopic arrangement of fibers in the medial lemniscus, localized lesions cause selective loss of fine-touch sensations limited to specific body regions.
The third-order neurons of the pathway are located in the ventral posterior lateral (VPL) nucleus of the thalamus and project to somatosensory areas of the cerebral cortex (
Fig. 7.5 •Fig. 7.5 Diagramofconnectionsfromthesomatosensoryreceivingnucleiofthethalamustothesomatosensorycortexoftheparietallobe.Notetheparallelflowofdifferenttypesofsomatosensoryinformationthroughthethalamusandontothecortex.CS,centralsulcus;S1andS2areprimaryandsecondarysomatosensoryareas,respectively.Note:collectively,areas3a,3b,1,and2arereferredtoasS1.
The dorsal column–medial lemniscus pathway conveys information about fine-touch and vibratory sensations. This information is critical for many of the discriminatory tactile abilities we have. For example, spatial acuity is lowered by damage to this pathway, and the ability to identify objects by their shape and texture can be lost by damage to this pathway. Clinically, one may test for impaired graphesthesia, or the ability to recognize letters or numbers traced on the skin, or for loss of the ability to tell the direction of a line drawn across the skin. Importantly, some tactile function remains even after complete loss of the dorsal columns, and awareness and localization of nonnoxious tactile stimuli can still occur. Thus at least some of the information carried by the dorsal column pathway is also conveyed by additional ascending pathways. In contrast to the severe deficits in discriminatory touch sensation, cutaneous pain and temperature sensations are unaffected by lesions of the dorsal columns. However, visceral pain is substantially diminished by damage to the dorsal columns.
Trigeminal Pathway for Fine-Touch Sensation From the Face
Primary afferent fibers that supply the face, teeth, oral and nasal cavities, and cranial meninges synapse in several brainstem nuclei, including the main sensory nucleus and the descending nucleus of the trigeminal nerve.
The pathway through the main sensory nucleus resembles the dorsal column–medial lemniscus pathway. This sensory nucleus relays tactile information to the contralateral ventral posterior medial (VPM) thalamic nucleus by way of the trigeminothalamic tract. Third-order neurons in the VPM nucleus project to the facial area of the somatosensory cortex.
Proprioceptors provide information about the positions and movement of parts of the body. In addition to being used for local reflexes (see
Chapter 9), this information has two main targets, the cerebellum and the cerebral cortex. The cerebellum uses this information for its motor coordination functions. The information sent to the cerebral cortex is the basis for conscious awareness of our body parts (e.g., position of our hand), which is referred to as kinesthesia.
The major pathways by which somatosensory information is brought to the cerebellum are shown in